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Controls on the Lateral Variation in Structural Style along an Evaporite-Influenced Normal Fault Array, Halten Terrace, Offshore Mid-Norway

Wilson, Paul *1; Elliott, Gavin 2; Gawthorpe, Robert L.3; Jackson, Christopher A.2; Michelsen, Lisa 4; Sharp, Ian 5
(1) Rock Deformation Research Ltd, Leeds, United Kingdom.
(2) Department of Earth Sciences and Engineering, Imperial College, London, United Kingdom.
(3) Department of Earth Science, University of Bergen, Bergen, Norway.
(4) Statoil, Harstad, Norway.
(5) Statoil Research Centre, Bergen, Norway.

The Halten Terrace is underlain by a rheologically weak Triassic evaporitic package, resulting in the development of vertically decoupled normal fault systems during Middle Jurassic to Early Cretaceous crustal extension. The fault systems that bound the eastern margin of the Halten Terrace show considerable along-strike variation in structural style. Mapping of faults and key seismic horizons on 3D seismic data, analysis of throw variations along faults, and seismic attribute analysis to identify likely evaporite facies, allow us to constrain the factors that influence the evolution of rift-related structural styles in this evaporite-influenced setting.

Four structural domains are identified along the fault system that bounds the eastern margin of the Halten Terrace, and from north to south these are: i) dominantly thick-skinned normal faults that affect both sub-salt basement and supra-salt cover, ii) basement-restricted normal faults associated with partially decoupled, fault-propagation folds, iii) dominantly thick-skinned, relatively distributed normal faults, and iv) dominantly thick-skinned, relatively localised normal faults. An abrupt change in structural style in the north of the study area is controlled by a fault domain boundary associated with a basement-involved, thick-skinned, NE-striking fault. Measurements of summed throw and estimated strain across the fault system show that the amount of throw and strain accommodated by the basin margin fault system increase towards the south, corresponding to a change to more localised faulting. The thickness of the evaporite package varies across the study area, with thickened areas occurring in the hangingwall areas of NE-striking, basement-involved, fault systems. However, variations in salt thickness do not correspond spatially to variations in structural style. Wireline logs from wells that penetrate the evaporite package, and volume attribute analysis of 3D seismic data, suggest marked facies variations in the upper of the two evaporite units. For example, a change in seismic facies from high-amplitude, low variance to low-amplitude, high variance corresponds to a change from dominantly decoupled to dominantly thick-skinned faulting. We conclude that the sub-evaporite fault template, the amount of strain accommodated across the fault system, and facies variations in the evaporite influence structural style, but variations in evaporite thickness are not large enough to influence style.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California